In the past various methods and apparatus have been conceived to manufacture headed articles having tubular shanks, such as rivets, in a single die. Historically, extruded semi-tubular rivets were first produced in large scale in multiple die headers. There are inherent difficulties with the single die approach resulting from the fact that metal flow is required at two locations, namely the extrusion at one end of a sheared blank to form the tubular shank and upsetting at the other end of the sheared blank to form the rivet head. Elaborate mechanisms have been required up to the present in order to promote a constant volume of metal flowing into the extruded tubular shank as distinguished from the upset head. Otherwise there will be unacceptable variations in shank length and head dimensions from rivet to rivet under production conditions. In many known cases, representing the majority of semi-tubular rivet capacity actually in use, these mechanisms have been added to the type of machines generally referred to in the art as single die double blow headers.
A double blow header in general terms includes a die block through which stock is fed by intermittently driven food rollers to a timed shearing mechanism whereby a blank is sheared off, transfer mechanism whereby the blank is positioned in front of a forming die, and a header slide carrying forming tools whereby the blank is given two heading blows before the next shearing cycle takes place. The header slide is reciprocated in the usual manner and usually includes two heading tools mounted on a shifting mechanism which alternately presents the tools to the blank. In a double blow header, the arrangement is such that the header slide makes two strokes to one stroke of the shear and transfer mechanism thereby providing the so-called double blow arrangement.
Machines of the double blow type were originally developed for the production of headed blanks where the material to be upset thereby forming the head corresponds to an amount difficult to upset and control evenly if the forging or upset action is not divided between a first and second blow. This generally considered to be the case when the volume of material required to form the head corresponds to a length of blank 2.5 times or longer than the diameter of the cylindrical blank. The great majority of small cold heated screw blanks fall into this category where two blows are required and it is in the area of screw blank production that the two blow header finds nearly universal application. However, the great majority of headed articles having a tubular shank, which in this application is called semi-tubular rivets even though the invention is useful for the production of other tubular or polygonal barreled hollow upset parts in addition to semi-tubular rivets properly called, have a head with less than the volume of material requiring two blows for good upsetting practice. This is to say that the head of semi-tubular rivets can generally be formed quite effectively in a single blow. In the case of single die double stroke tubular headers described in the prior art, one of the two blows performs the function of forming the sheared blank in such a way as to establish the proportion of material flowing into the extruded shank as opposed to the head and other blow completes the operation. In many of the prior structures (U.S. Pat. No. 3,720,968 and U.S. Pat. No. 3,200,424) the first blow performs the extrusion operation and the second blow performs the upsetting operation. Thus it can be appreciated that in these two methods which are currently widespread and important in the industry, the cold heading machine is essentially operating as a single blow header insofar as the head upsetting operation is concerned.
One of the most expensive mechanisms of two blow headers is that which shifts the heading tools in order to present them alternately to the blank during each machine cycle. It is also the mechanism which tends to wear out first during the useful life of the machine and hence its use entails a substantial ongoing maintenance expense as well as great skill on the part of the operators which adjust it.
Typically double blow cold header configurations for the production of semi-tubular rivets such as those covered in U.S. Pat. Nos. 3,200,423; 3,200,424 and 3,720,968 rely upon the motion of the heading slide in order to provide the motion which effects the extrusion of the tubular shank. These mechanisms all suffer from the great drawback that the tool which contacts the blank and over which the material extrudes, and which will be referred to as the extrusion pin, inevitably is driven with its greatest velocity at the instant of initial contact, and its velocity relative to that of the mass of the sheared blank decreases as the extrusion proceeds. This is exactly the inverse of the ideal situation, and the high shock loading on the extrusion pin at the moment of initial contact is a major factor in limiting the maximum production speed which is practical for cold headers of this type. At high speed the extrusion pin tends to be deflected off center, and under production condition cases occur when it will actually collapse under compressive stress.
Claims have been made (U.S. Pat. No. 3,540,255) for forming hollow metal articles in a single fixed die with a single blow. However in these claims the extrusion operation takes place when the heading punch is closed against the die thus confining the upset blank in such a way that the shank material will flow back over the extrusion pin when advanced against the shank. This previously disclosed technique has the disadvantage that a heading blow delivered by a punch mounted on a slide driven by a crank shaft and pitman arm permits only a very restricted portion of the machine cycle to be dedicated to the forward movement of the extrusion pin. This is because the header punch remains closed only instantaneously during the machine cycle, and any attempt to drive the extrusion pin forward and continue extruding once the heading punch begins to move away from the heading die following the forward dead center position results in the shank bulging under the head which is at the junction of the straight shank and upset portion of the piece. This is unacceptable and must be avoided. In an ordinary single blow design the extrusion motion can be initiated effectively by advancing the extrusion pin at a point in the machine cycle slightly before the forward dead center position of the heading punch and the motion can be continued until just passing the forward dead center position. During this short interval the portion of the shank being formed by the header punch is contained either by the punch form or what amounts to a "friction hill" which resists further flow of the upset material between the closely spaced parallel or nearly parallel surfaces of the header die and header punch. Hence during this short interval the pressure generated by the extrusion operation is unlikely to cause large variations in the amount of material formed in the upset portion of the work piece corresponding to the head. Subject to the above mentioned restriction the hollow shank length of the article produced will not vary excessively.
Unfortunately this simple mechanical expedient places a severe limitation on the maximum production rate which can be achieved. The maximum accelerations of the extrusion pin must be limited and this limits the cycle speed when driving the extrusion pin by a rocker arm movement driven from the header slide. In an attempt to reduce the impact load on the extrusion pin a cam driven motion is often provided for the extrusion pin. Since cams fail if required to deliver a stroke in too short an interval of time, this approach also severely limits the machine cycle times which can be attained.